In principle, it is already known to provide in hydraulic brake systems of motor vehicles a pump device which when required increases a hydraulic pressure in the brake system in order to carry out an automatic braking operation, for example, an emergency braking operation, independently of a brake pedal actuation by the driver. Generally, known pump devices have a piston pump which is driven by an electric motor so that a rotation movement is converted into a translation movement. The omission of conversion of the rotation of the motor into an oscillating piston movement, which conversion involves friction, leads to a higher degree of efficiency and a compact construction of the pump device being ensured.
The piston movement can be controlled by changing the amplitude and frequency thereof independently of each other. These two degrees of freedom in terms of the control enable favorable operating ranges. An advantageous development has been found to be the construction of the linear drive as a reluctance drive which can be produced in a simple, precisely controllable and cost-effective manner. The linear reluctance drive has a longitudinally movable armature and a stator which is secured to a housing and to which current can be applied. If current is applied to the stator, there is produced a magnetic field which leads to the armature being urged into a neutral position in which the magnetic resistance for the magnetic flux of the magnetic circuit is minimal. There is associated in this instance with the armature at least one resilient element which acts on the armature counter to this drive force of the reluctance drive in order to move the armature out of the neutral position. If no current is applied to the stator, therefore, the resilient element urges the armature out of the neutral position into a starting position. If current is applied to the stator again, it urges the armature magnetically counter to the force of the resilient element into the neutral position. As a result of corresponding control of or application of current to the stator, the travel and the frequency of the movement of the armature can consequently be adjusted independently of each other. The armature is in this instance connected to a pump piston which is supported in an axially displaceable manner, for example, in a hydraulic chamber in order to pump the medium located at that location.